This invention relates to the cleaning of workpieces, particularly but not exclusively to the cleaning of printed circuit boards upon which electrical components have been soldered.
When such circuit boards emerge from the soldering process they have thereon contaminants such as flux residues and the like which have to be removed by using a liquid cleaning solvent. One of the solvents used widely for such a task, namely stabilized, azeotrope of CFC-113 and methanol available commercially as Dupont Freon TMS, is totally safe from fire or explosive hazard. However, this solvent is known to deplete the ozone layer and will most probably become banned throughout the world by a large number of countries who signed the UNEP agreement in 1987.
New solvents or cleaning agents are being tried, not as direct substitutes, but as alternatives. Among these alternatives are Terpene solvents CYCLO-ALIPHATIC and Petroferm EC-7 and other LIMONENE based solvents which unfortunately have a low flash point and when sprayed as a cleaning agent for metal parts, electronics circuits and the like in air, the risk of a potential fire or explosion becomes very real.
Previously, solvents which are safe have been located in open top tank type cleaning machines, some of which may have had cooling coils to contain the solvent vapours within the open top tank. Other machines were made longer, some with multiple stages and a conveyor system consisting of either baskets on a transporting device or with a conveyor belt which would convey parts down into the open top tank into the vapour zone, sometimes also into the liquid cleaning solvent and which also may have been boiling and in some cases may also have had an improved mechanical cleaning effect with ultrasonic agitation of the solvent and/or with mechanical pumping devices and nozzles to spray on the workpiece which result in improved cleaning. The tops of these machines are sometimes closed to reduce solvent losses with entrance and exit openings for the workpieces.
With the recent advent of electronic circuits such as those containing surface mounted devices (SMD) and the like which are mounted quite flush to the circuit board and which are known to present cleaning difficulties, equipment has been developed with high pressure sprays up to the order of 200 psi (12 atmospheres) pressure and even higher. In order to contain the spray and resulting increased losses of the solvent associated with such high pressures, systems exist for transporting metal parts and the like in a basket transport system through liquid seals; see for example U.S. Pat. No. 3,120,853. More recently, vapour degreasing of workpieces through similar machines without transporting baskets but with an endless mesh belt transport conveyor with liquid seals for containment of the high pressure spray, whereby the high pressure spray and associated vapours are contained in a central zone and the entrance and exit zones are generally quiet, thereby resulting in high containment of the solvent in the machine.
The use of potentially flammable or explosive solvents in such cleaning machines is hazardous. This is particularly true for the central zone where the spray nozzles are located because of the intimate mixing of the liquid droplets in the air contained in that zone.
It is an object of the present invention to provide a method and apparatus which reduces this inherent danger.